The coolpix like others
digicams fire 2 times his strobe when shooting a photography with flash. Then
classic slave strobe unit can't work correctly, because they fire on the 1st
flash on the digicam, and not on the second as they should do.
I've tried all the available modes on the cp880, even the "full manuel"
mode; the preflash is always used !

This problem can be solved
with somes specifics strobes units dedicated to dual flashs camera (see srelectronics.com),
but it's definitly not the cheapest way!
The solution i choosed, is to build myself my optical trigger module to drive
an old strobe unit i have (Sunpak auto 240). It's only cost time, but not too
much money !

I found on the internet
(on the dpreview
forum especially) a lot of contact with people who already have done this
kind of electronic module. For more details, please visit the dedicated
page.

Caution
! The schematics presents in my pages are shown without any warranty.
The only thing i could say is that my 2 schematics works with my strobe unit.
The "dual flash" version works with both my strobe unit and my coolpix
880. I never had try the other schematics...

"Single
flash" schematic (version 2)

Because,
first all my 74HC4017 has burned out (you will anderstand later on this page),
i started to debug a "simple" version of the module, only compatible
with camera wich doesn't use preflash.
It works correctly with a quite nice sensitivity in differents light conditions.
This slave fire even if the camera is far from 7 or 8m and not orientated in
direction of the slave.

This schematics have been tested with success in different
ambiant light conditions.

Adaptation
for newer flash internally IGBT drived (ex: METZ 32Z 2) :

For some recents
strobes units, high voltage is no more present on trigger pins, because some
built in electronic drive this high voltage with thyristor or IGBT.
I recently bought a "METZ 32Z 2" witch is IGBT drived. To adapt my
module to it (Thyristor need high voltage to start conducting) i replaced the
thyristor part of the previous schematic by a "bipolar" version of
it, with the thyristor simply replace by a very common NPN transistor. I don't
know if this adaptation will also work with thyristor drived strobe unit...

Explanations,
principle of operation and comments :

Optical
sensor

The phototransistor current
increase if ambiant light luminance increase too. The higher is the luminance,
and the higher is the voltage on tpA.

When i first watch the tpA
voltage with the oscilloscope i saw 2 parasitics frequencies at 100Hz and 200KHz.

200 KHz
parasitic :

The 200KHz generate a lot
of counting errors, but disappear after i supply my module with batteries rather
than by programmable AC power supply.

With AC programmable
power supply :

With batteries supply,
the 200KHz desappears, but 100Hz still exist :

100 Hz parasitic
:

I
first search on electric phenomena this 100 Hz parasitic. It can't be something
relative to AC power line because the module is supplied by batteries. The
only 50Hz or multiple present around the module was the light !

When the phototransistor
is exposed to light comming from indoor classic lamps (AC line 50Hz 220Veff
in France), it react in relation with light intensity witch is variable in
this case. A lamp haven't got any polarity, then with 50Hz AC line a maximum
voltage appears 2 times per period, then the light intensity frequency is
= AC line frequency * 2.
50 Hz * 2 = 100 Hz.

When the phototransistor
is exposed to light comming from a pocket lamp using batteries, the 100Hz
totally disappears.

module
exposed to classic AC line lamp :

module
exposed to pocket lamp :

Resistor
setting :

The serial resistor with
phototransistor define the gain of the sensor. For the same current in the phototransistor,
the bigger is the resistor value, and the higher is the voltage on tpA.
1 kohms with 6V supply voltage is suitable for most common light condition,
however for a very dark ambiant light (outdoor inght shots without any moon
light) 10 kohms generate a higher sensor sensitivity, so remote distance between
camera and the trigger module can be increased. Don't think it also increase
sensitivity for brighter conditions. In fact the voltage on tpA becomes so high,
that it clamp to vcc. The sensitivity is then totally null !
To finish, lets say that 10 kohms is optional, but it could help...

High pass
filter

principle
of operation and calculation

A high pass filter is used
to activate the counter clock input only when a pulse appears with a rise time
short enough. Then the system is totally independant of the mean ambiant light
intensity and will detect each time the light is changing quickly. (It doesn't
start if you simply switch on the room light for instance).
The other interest of that, is to filter the 100 Hz parasitic coming from AC
line lamp.

When
Vin doesn't change (or slowly enough), C is charged to Vin, then Vout = 0.
If Vin change quickly, C doesn't have time enough to be charged, the voltage
change is present on Vout. However due to dividor bridge R1&R2, the pulse
on Vin is reduced on Vout in respect with R2/(R1+R2) ratio.
The cutting frequency of this filter is defined by 1/(2*pi*R*C). For R, 2 cases should be considered :

Vout < vbe of the
transistor connected on output : ib = 0, R = R1+R2. It's the most common
case, especially if there is no light pulses (flash) present and also when
only parasitic 100Hz is here.

Vout >= vbe : ib increase,
then the current inside C is the sum of current in R2 and ib. Vout is almost
fixed = vbe. In this condition, i think the R*C to be considered is
R1*C. The cut frequency is higher. C is charging quicker.

The equation to consider
is "1/(2*pi*(R1+R2)*C)" because when Vout reachs vbe, it means
that detection is effective (too late to filter or not). And the purpose of
filtering is to define wether or not a voltage variation should be detected.

100
Hz filtering

tpA
: 100Hz Before filtering

tpB
: 100Hz after filtering

I started with a first version
with no R2 resistor. In that case an infinite value can be substitute in the
cut frequency equation, wich indicate that the filter won't filter anything
! (there is no current to charge C until Vout reach a transistor vbe)

Other
parasitics filtering

Some other parasitic signals
needs to be filtered. When anything is agitated in front of the optical sensor,
the light that reach it may vary quickly. If the high pass filtering cut off
frequency is too low, somes pulses could appears on Vout (tpB). The next oscilogram
show an example of that, with a pen agitated in front of the sensor and a filter
made with 10 nF, 1K and 18K.

After this
trial, i choosed a smaller capacitor (3.3nF) to increase the cut off frequency,
and then decreassing the module sensitivity to parasitic triggering.

with
coolpix flash

However,
with 1k, 18k and 3.3nF values, the cut off filtering frequency is higher enough
to filter parasitic, and lower enough to pass the coolpix flash pulse. Thoses
pulse are really quicker than parasitic, as you can see on both oscillograms
below. Please note the very high positive slope on the pulse rising edge.

Schmitt
trigger

HC type ICs power consumption
is very low, except during logic state transition. If input voltage slope is
low, the period of time when the input voltage is still near the input threshold
voltage is long. During this critical period ("critical area" in the
figure below), the IC may trigger several times because it could exists some
ambiguity between input voltage and threshold (is the voltage bigger than the
threshold or not ?).

I said before, that power
dissipation on HC type ICs (MOS inside) is not low during logic state transition.
So when multi-triggers appears, the dissipated power increase so much that definitive
damages could happens. I destroyed, by this mean, four 74HC4017 counters !

The solution is to avoid
ambiguity. Schmitt trigger are devices build for this. They have a variable
input threshold voltage. When the input voltage reachs the threshold one time,
this one decreasse immmediatly and the IC trig just one time (as it is supposed
to). The threshold won't change until the input voltage start decreasing and
reach again, but in the opposite direction, this new threshold. Then the threshold
come back to his first value.

Without
Schmitt trigger :

With
Schmitt trigger :

So be
carefull !!! With HC type ICs, please consider seriously your rise and fall
time on inputs ! If the time is bigger than specification value, add a schmitt
trigger before.

Counter

Nothing special to explain
here about the way it works. It's a generic device. If you discover 74HC4017
for the first time and if you wan't more informations about it, look the datasheet.
It's pretty well explained.

Some web surfer asked to
me "why don't use some flip flop in place of counter?". Answer : Because
i like the idea to be able to trig a strobe unit on the third flash (a single
flipflop would be limited to the second). In term of number of components, i
think that there is no gain to replace counter with filpflop. In both case you
will need one IC package (14 or 16 pins doesn't change anything).

Reset timer

To avoid unwanted triggering
due to other cameras shooting in the same place, or some hypothetic strong electrical
perturbations, an "auto-reset" have been added. For people who could wonder
if this kind of supplementary logic is really necessary, let's imagine a module
that generate a systematic auto-reset after the counting reach 2. What happen
if someone else takes a picture with his own camera witch generate only 1 flash
? I'm afraid that everything is then shiffted. The preflash will be considered
as the second flash, the strobe is fired (it is not supposed to be). The real
second flash is considered as the next first and the external strobe is not
fired when needed !

My auto-reset happen if
:

2 consecutives flashs
are not detected during a certain delay.

Counter is not already
reseted (to avoid a continuous reset at start up).

It's
made by some logic gates that generates a rising edge on counter "MR"
(master reset) pin if :

counter "0" output
is low (witch means that counting is > 0)

multivibrator pulse is
ended (or is not started) = logic 1

3 gates are used. It could
have been done with a lower number of gates, but the NAND with schmitt trigger
used for the counter clock input, is supplied in quad gates package. So why
don't using it ?!
The multivibrator start his pulse on the first flash (rising edge of counter
"1" output), and finish after a delay determined by a RC resonator
on "R/Cext" & "Cext" pins. With actual values,
the measured pulse delay = approx. 200 mS
A schtocky diode is present to avoid IC damages during C discharging.
The timer trigger input "1B" doesn't need external schmitt trigger, because
one is included inside the IC and signal is comming from another HC type IC.The resistor on the
1Q bar output is there to enable the module to work without any multivibrator
plugged into his socket, for debugging purpose without any counter reset.

I was first thinking to
a NE555 but logic edges were opposites of my need and i like the idea to have
all logic ICs in the same family, so the timer is a 74HC221.

Caution
! The theorical pulse duration for the RC choosed should be 700mS, but due to
capacitor leakage probably, the measured value is about 200 mS. It could be
interesting to check the multivibrator pulse duration with another capacitor
type. The duration
must be greater than interval between 2 flashs (approx. 100mS).

Thyristor

To drive common
strobe

To fire a strobe
unit, old cameras simply shortened strobe trigger mains pins. The problem
here is that voltage present on thoses pins is usualy very high. A bipolar
transistor won't resist. So thyristor (SRC) is often used for this commutation.

A sensible thyristor
is used because his low gate current enable direct counter output driving.
HC type ICs can drive 8 mA.
1 kohms serial resistor set 5mA (with vcc = 5v) maximum current, witch
is 20 times more than expected thyristor gate current. Then commutation
is quicker and so, more suitable to high voltage commutation.

The TIC106M is working
correctly with my sunpack 240 (40V is present on his trigger pins),
but i suppose (i've never checked it) that it could drive higher voltages.
Some strobe unit seems to present 200V.
TIC106M component is specified to drive several hundred of volts. (see
datasheet)

I recommand you
to place the thyristor as close as possible to your strobe shoe (visit
the SAM page for explanations). I placed it directly below the shoe on
my module.

To drive more
recent strobe unit

In few new strobe
unit, the high voltage is commutated inside the unit. Then high voltage
is not output from the unit. Instead trigger pins are controlled with
low power signal. I won't explain in detail this driving approach, because
i haven't got too much informations about it (tell me if you know something
about it).

Because no high
voltage exist on input trigger pins, thyristor can't be used (it need
suitable gate conditions to start conduction, but also sufficiant voltage
and current between his anode to cathode).

Bill
of materials (+Datasheets)

Cost

How to use the
module with a Coolpix 880

Turn on your
strobe unit, your trigger module and set your camera to flash mode. Then shoot.
Easy no ?!Yes
shooting with strobe light present on your picture is easy, but unfortunatly
it's really more difficult to reach the step where your pictures are always
well exposed and focused.
A lot of questions, but not a lot of answers !

What
happen during preflash ? I
don't know !
I don't think it adjust the exposition settings. So it's a mistake !I probably missed something.
I already have success with this technic, thus my slave strobe unit is only
firing on the second coolpix flash.

Autofocus
:

The
most difficult thing to manage is the very capricious coolpix880 autofocus.
In dark conditions, the light is quickly not strong enough for the coolpix to
be able to auto focus or on an unwanted subject. So your picture could be almost
well exposed but with a very bad focus. One way to success is to find a subject
exposed to light for the coolpix to run his autofocus on it (half press the
button), and then to move your digicam back to your main subject (without releasing
the button to keep the adjusted focus) before shooting. Both subject should
be at an equivalant distance.
Perhaps an other solution to investigate would be to use an external portable
lamp to illuminate the subject during auto focus.

Exposure
:

The
other difficulty is to adjust the exposure (but i really prefer to fight with
exposure problems rather than with auto focus problems). There is no closed
loop to automatically adjust it as it could be found on usual camera-strobe
pair.With
optical trigger module, you will have to adjust camera and strobe separately.
Adjusting shuter speed on the camera doesn't give anything, because in the case
of flash photography, the exposure time is not defined by the shutter speed
but by the strobe light duration which is very shorter (few µS) compared
to the shutter speed. (when the strobe light is finished the camera shutter
is still opened but in front of a dark image). The only way to adjust exposure
on the camera is to adjust aperture. Unfortunately the coolpix 880 has got only
two aperture settings : The minimum value and the maximum. It's not sufficiant
to adjust correctly the exposure.The
second solution is to adjust the strobe intensity. It could be done directly
on the strobe setting if it has been designed for. If it doesn't, the last solution
is to choose the strobe location that take care of the intensity you need. If
you need a strong light, strobe could be placed far from walls, with light in
direction of the subject. On the other hand if you need low light, strobe can
be moved back close to the wall, in the opposite direction of the subject and
so on...The last solution to adjust
exposure is to adjust distance between strobe and subject.

At this point
let's make a summary about exposure adjustement possibilities :

camera aperture setting

strobe settings (if it
has some)

strobe location (distance
to the subject, light direction, distance from wall or any obstacles...)

The technic i use the most
is the strobe location. This is the method that give you the best adjustement
range. To use it, you will need to build your own experience, but with time
it's possible to learn how to predict the amount of light that will result from
each strobe location. However several shoots are often necessary for each new
subject to optimize the exposure adjustement.